Bright dip



' with a pH above 5 Neither the Patented Apr. 18, 1939 UNITE STATES MUSE PATENT OFFICE BRIGHT DIP Richard 0. Hull, Lakewood, Ohio, assignor, by

' mcsne assignments, to E. L du ,Pont de Nemours a Company, ration of -Delaware n, Del, a corpo- No Drawing. Application December 17, 1984,

Serial No. 757.880

2.. claims. (c1. 14H) This invention relates to brightdips for zinc plated articles, and is particularly directed to bright-dips, and to processes of bright-dipping, which employ an acidic aqueous solution containing about two to eight per cent hydrogen peroxide, the aqueous solution having a pH above about one.

In addition to their lowcost, electrodeposited 'zinc coatings display many characteristics which should cause them to be particularly desirable as protective and ornamental finishes. Although zinc is not very resistant to corrosion, it does not accelerate the corrosion of iron or steel as do such metals as copper and nickel. On the contrary, zinc being higher in the electromotive series, will protect iron or steel against rusting even afterappreciable areas of the base metal are exposed. Polished zinc metal moreover, has a pleasing metallic lustre with a slightly blue hue.

Despite their numerous advantages over many commonly used coating materials, electrodeposited zinc coatings have not enjoyed the use which they deserve because they do not possess, and even if polished, do not retain a pleasing appearance. The known methods of electroplating zinc almost invariably result in dark, discolored plates. Even when the plates at first have a fairly satisfactory appearance, they soon become dark and dislow cost I thus obtain a finish which is acceptable in practically any field that requires a protective metallic finish. i

The bright dip compositions of this invention are composed of hydrogen peroxide in acidic, aqueous solution. The peroxide concentration may vary widely, but the best results are obtained with from about two to eight per cent 1120:, by'

go weight. The peroxide is used in an acidic solution about five-tenths, tho, as will hereafter appear, lpreferto use from about pH 0.5 to pH 3.5 or, more specifically, from about "pH 1. to pH 2.5.

hydrogen peroxide nor the acidic materials serve alone as a satisfactory brightdip. Alone, hydrogen peroxide may remove a slight amount of stain, and alone an acidic solution, within the pH range here used, has little effect. Apparently there is some synergetic action between hydrogen peroxide and acidic materials which causes an acidic peroxide solution to exercise an unexpected passivatingand brightening action,

The hydrogen peroxide solution may be acidified with any desired material of acid reaction. I prefer to use sulfuric acidfiboth because it is relatively inexpensive and because it leads to excellent results; I may, however, use inorganic acids such as hydrochloric acid, phosphoric acid, and carbonic acid, or organic acids such as acetic (acid, lactic acid, and citric acid. Very weak acids such as carbonic acid and hypochlorous acid are none too satisfactory for, while they operate as do stronger acids, they are effective to a much smaller degree. Some acids, such as hydriodic acid, are not economical because they cause a rather rapid decomposition of the peroxide. While I may use any acid, organic or inorganic, I prefer to use acetic acid or a mineral acid such as sulfuric acid or'hydrochloric acid.

Instead of using acids, I may acidify the peroxide solutions with acidulous salts such as ferric chloride, potassium per sulfate, and sodium bi-,

sulfate. While any acidulous salt may be employed, certain ones, such as iron or copper salts, are none too satisfactory because they accelerate the decomposition of hydrogen peroxide. It is also noted that, generally, I prefer the bright-dip not to contain a compound the cation of which is a metal below zinc in the electromotive series.

The ratio of hydrogen peroxide to acidic material is of great importance. when sulfuric acid is used as the acidifying m terial, the lay- -drogen peroxide (H1O: by weight) to acid (H2804 by weight) ratio should not be far outside the ratios of four to one and forty-eight to one. If

substantially larger or smaller amounts of acid are used, the results obtained are not as satis-v factory as may be desired. There are numerous other factors such as cost, stability, etc. which make the above indicated range an optimum.

The hydrogen ion concentration of a given peroxide solution, therefore, must fall within certain limits if the optimum peroxide-acid ratios are to be used. With a two per cent H202 solution, for instance, the E804 content may vary from about one twenty-fourth per cent to about onehalf per cent. These limits correspond to a hydrogen ion concentration range of about pH 2.2

to pH 1.4. with an eight per cent H: solution,

tration H2O: by weight) by the following range:

1. Bright-dip pHglog 7 %f (48 to 1 ratio) 2. Bright-dip pHglog 7 to 1 ratio) The first expression is substantially equivalent to saying that the peroxide-sulfuric acid ratio is forty-eight to one, and the second expression substantially equivalent to saying that the peroxide-sulfuric acid ratio is four to one. These expressions thus represent the limits found'by experiment. It will be understood that the equations are not mathematically exact definitions of the relationships existing. The pH of a solution is a function of the dissociation constant of the acid at the particular concentration, and in the equations an average dissociation constant is used. The variation from the exact relationship is not great, and, as great exactitude is not required, the definitions are suiiicient for my purposes.

Comparing computed pH values with those given above, it is seen that with a two per cent peroxide solution the pH may range from to: rrgie w -14 These values, then, agree with those above given. At a peroxide concentration of eight per cent, the above values are pHl. to pH 1.7 while-the computed values are pH .8 to pH 1.7. .The error in the high acid limit of the eight per cent peroxide solution is, therefore, only two-tenths pH. and this is about as great an error aswill be encountered.

I prefer that the pH be no lower than pH 1., and expression 2. may be revised to meet this condition as follows:

80 3o Bright-dip ling 108 9 77?,

According to this, the high acid limit for an eight per cent peroxide solution ispH 1., and for a two per cent peroxide solution is pH 1.6.

'- ing to Equation 4, of about pH 1.9 and its actual- More specificallyl prefer to use ahydrogen peroxide-sulfuric acid ratio of about sixteen to one.

This may be expressed for various peroxide concentrations: 4. Bright-dip pI-I 16o A preferred two per cent peroxide solution, then, contains aboutone-eighth percent sulfuric acid (HgSOd by weight), and it has a pH, accordpH is about pH 1.82. A preferred four per cent peroxide solution, similarly, contains one-fourth per cent sulfuric acid (H2804 by weight), has a calculated pH of pH 1.6, and has an an actual pH of about pH 1.61. A preferred eight per cent peroxide solution, similarly, contains about one-half per cent of sulfuric acid (H2804 by weight), has a calculated pH of pH 1.3, and has an actual pH of about pH 1.40.

The pH may be determined in any desired manner: for instance, by electrometric'methods. ltwill be readilyunderstood that in measuring the pH of a given solution with color indicators, an acid solution should be made up to the required strength and tested before adding the peroxide. If the peroxide were present, it would tend to bleach and decolorize the indicators and thus falsify the results obtained.

The generalizations above given for sulfuric acid hold to some extent for other acidic materials, and I shall use the approximations of the above equations as one generic mode of expression. A highly ionized acid, hydrochloric acid, operated very satisfactorily with four per cent peroxide and at a pH 1.6 (according to 4).

A weak acid, such as acetic, which is only slightly ionized, does not need to be used in as great a concentration as the pH determined from the above equations would indicate. With a four per cent peroxide solution, the pH values found most practical ranged from about pH 2.95 to about pH 2.68. This amounted to an acetic acid content from about four-tenths to one and fivetenths per cent by weight. The expression 1 may be revived to include such weak, acidic materials within the range from expressions 1 to 2 or 1 to 3, thus:

4 00 Bright-dip pI-Ilog 7 Extremely weak acidic materials, such as carbonic acid, cannot be used to obtain a low pH. I include such materials broadly, however, when it is desired merely to use a peroxide solution with a pH above pH 1. or pH 0.5.

It will thus be understood that the above equations and considerations represent approximations, and that some allowances are preferably made according to the specific nature of the acidifying material used, the conditions of use, and other similar factor The following specific examples are given more completely to explain my invention:

Example 1.A steel article was zinc plated in known manner in a conventional bath of the following composition:

Grams per liter NaOH 38 NaCN 80 ZnO Gelatin: I 1

After the article was zinc plated, it was rinsed in water and then dipped into a bath of the following composition:

Hydrogen peroxide 4.% H202 by weight Sulfuric acid 0.25 H2804 by weight Hydro en peroxide 2.% H20: by weight Sulfuric acid 36% also. by weight Very ood results were obtained. With the same amount of peroxide and from to 95% H2804, good results were obtained. The time required satisfactorily to brighten the plates varied from about ten to twenty seconds.

Example 3.--The process of Example 1 was fol- :cwed with a bright-dip of the following composi- Hydrogen peroxide 8% H2O: by weight Sulfuric acid 0.5% H2504 by weight Very good results were obtained. Using the same amount of peroxide, bright-dips using from $4; to 2% H2804 were tried with good results.

Example 4.--The process of Example 1 was folcwed with a bright-dip of the following composi- Very good results were obtained. Using the same amount of peroxide, bright-dips were tried with from 0.4% to 1.5% acetic acid with good results. I

While I have mentioned a specific type of plating bath in the examples, I do not intend to be limited thereby. I may use my bright-dips on articles plated by any method. In general, however, the most satisfactory results are achieved when as smooth and bright a plated article as obtainable is treated.

As above described, my process includes plating an article, rinsing it, and exposing it to the action of one of my bright-dips. I sometimes flnd it desirable to interpose an acid dip treatment between the rinsing and bright'dip treatments. By subjecting the plated article to the action of a dilute acid, some foreign materials, such as residues of the plating solution, may be removed. The life of the bright dip may be lengthened, as

r the plated article carries less material which might dipped into a solution of C. P. sulfuric acid.

After a few moments the article was removed from the dilute acid, rinsed in water, and then exposed to the bright-dip of Example 1. The

plated article had an excellent finish. This procedure was repeated until the bright-dip lost its effectiveness. Similarly, following the process of Example 1 until the dip was exhausted, it was found that the use of the preliminary acid dip extended the life of the bright dip almost fifty per cent.

Instead of using sulfuric acid in the preliminary acid dip, I may use solutions of other acids and acidulous salts such as are above enumerated.

The concentration of the preliminary acidic dip may be varied widely, but it should not be so concentrated as to etch the zinc finish. The preliminary dip is characterized by containing a substance of acid reaction such as an acid, organic or inorganic, or an acidulous organic or inorganic salt. I prefer, however, to use dilute sulfuric acid. I may, if desired, omit either, or both of the rinsing treatments. If the second rinsing treatvl vuv ment is omitted, a small amount of acid ,will be carried into the bright dip with each article. Care must be exercised, in this case, lest the bright-dip become too strongly acid.

My bright-dip has a relatively long life, but care must be exercised lest some agent or condition be encountered which will accelerate decomposition.

I prefer to use relatively pure acids, because impurities such as are commonly found in commercial acids may cause decomposition of the bright-dip. The solution is preferably shipped and used in glass, glazed stoneware, hard rubber, or stainless steel receptacles. If lead, Monel metal, fiber, and similar materials are used to contain my peroxide bright-dip, a more rapid decomposition occurs.

I may, if desired, add a small amount of a known stabilizer for peroxide. The nature and use of stabilizers is well known to the trade and need not be described in detail. I have found small amounts of alcohol, sodium pyrophosphate and sodium stannate, glycerine, etc. quite satisfactory.

The peroxide dip may be sold in the form of a relatively concentrated solution of peroxide and acidulous material and a buyer can dilute the product to the desired extent. 0! course, it can be sold as a relatively dilute solution or it can be made up at the place of use.

While I have shown certain specific composition and processes, I do not intend to be limited thereby, the scope of my invention appearing from the following claims.

I claim:

1. A bright-dip for zinc comprising an acidic hydrogen peroxide solution with a pH from about pH 0.5 to pH 3.5. v

2. A bright-dip for zinc comprising an acidic hydrogen peroxide solution with a pH from about pH 1 to pH 2.5.

3. A composition for passivating and brightening an electrodeposited zinc coating, comprising hydrogen peroxide and an acid, the peroxide amounting to about two to eight per cent by weight, and the composition having a pH of the order of 4. a composition for passivating and brightening an electrodeposited zinc coating, comprising hydrogen peroxide and acetic acid, the composition containing about two to eight per cent of peroxide, and the composition having a pH of the order of 360 80 I pHglog m to P112101 0 W,

5. A composition for passivating and brightening an electrodeposited zinc coating, comprising hydrochloric acid, the composition containing about two to eight per cent of peroxide, and having a pH of the order of 360 pH log m to pHziogm m 6. A composition for passivating and brightening an electrodeposited zinc coating, comprising hydrogen peroxide and sulfuric acid, the ratio of H102 to H1304 being from abput forty-eight-toone to four-to-one, and the composition containing about two to eight per cent B102.

7. A composition for passivating and brightening an electrodeposited zinc coating, compristhe action of a bright ing about four per cent of hydrogen peroxide and about one-fourth of a per cent of sulfuric acid.

8. In a'process of passivating and brightening an electrodeposited zinc coating, the step comprising subjecting the electrodeposited coating to the action 01' a bright dip comprising an acidic hydrogen peroxide solution, the bright dip having a pH no lower than about pH 0.5.

9. In a process of passivating and brightening an electrodeposited zinc coating, the step comprising subjecting the electrodeposited coating to dip comprising an acidic hydrogen peroxide solution with a pH from about pH 0.5 to pH 3.5.

10. In a process of passivating and brightening an electrodeposited zinc coating, the step comprising subjecting the electrodeposited coating to the action of a bright dip comprising an acidic hydrogen peroxide solution with a pH from about pH 1. to pH 2.5.

11. In a process oi passivating and brightening an electrodeposited zinc coating, the step comprising subjecting the electrodeposited coating to the action of a bright dip comprising an acidic hydrogen peroxide solution which has a pH of the order of 12. In a process of passivating and brightening an electrodeposited zinc coating, the step comprising subjecting the electrodeposited coating to the action of a bright dip comprising an acidic hydrogen peroxide solution which contains about two to eight per cent peroxide which has a pH of the order 01 has a pH of the order of 360 80 pH glog %H2o to pHgiog m .14. In a process of passivating and brightening an electrodeposited zinc coating, the step comprising subjecting the electrodeposited coating to the action of a bright dip comprising hydrogen peroxide and acetic acid, the bright dip containing about two to eight per cent of peroxide, and the bright dip having a pH of the order of 15. In a process of passivating and brightening an electrodeposited zinc coating, the step comprising subjecting the electrodeposited coating to the action oi a bright dip comprising about four per cent of hydrogen peroxide,.and

about four-tenthsto one and five-tenths per cent of acetic acid.

16. In a process of passivating and brightening an electrodeposited zinc coating, the step comprising subjecting the electrodeposited coating to the action of a bright dip comprising hydrochioric acid, the bright dip containing about two to eight per cent oi peroxide, and having a pH of the order of 360 50 PH =10fl. '%H,O, t0 fin-:10! "W n a 17. In a process of passivating and brightening an electrodeposited zinc coating, the step comprising subjecting the electrodeposited coating to the action of a bright dip comprising hydrogen peroxide and sulfuric acid, the ratio of H202 to H2804 being about forty-eight-to-one to iour-to-one.

18. In a process of passivating and brightening an electrodeposited zinc coating, the step comprising subjecting the electrodeposited coating to the action of a bright dip comprising hydrogen peroxide and sulfuric acid, the ratio of H202 to H330; being from about iorty-eight-to-one to i'our-to-one, and the composition containing about two to eight per cent H202.

19. In a process of passivating and brightening an electrodeposited zinc coating, the step comprising subjecting the electrodeposited coating to the action of a bright dip comprising about two to eight per cent of hydrogen peroxide and the quantity of sulfuric acid required to make the Ham-H2804 ratio about sixteen to one.

20. In a process of passivating and brightening an electrodeposited zinc coating, the step comprising subjecting the electrodeposited coating to the action of a bright dip comprising about four per cent oi hydrogen peroxide and about one-fourth of one per cent of sulfuric acid.

21. In a process of passlvating and brightening. an electrodeposited zinc coating, the steps comprising subjecting an electrodeposited coating to the action of a dilute acidic solution, and subsequently subjecting the article to the action of a bright dip comprising an acidic hydrogen peroxide solution having a pH of no lower than about pH 0.5. a 7

22. In a process of passlvating and brightening an electrodeposited zinc coating, the steps comprising subjecting the electrodeposited coating to the action of a dilutesuliuric acid solution, and subsequently subjecting the article to the action of a bright dip comprising hydrogen peroxide and sulfuricv acid, the ratio of H202 to H2504 being from about forty-eight-to-one to four-to-one, the composition containing about two to eight per cent H202.

23. In a process of passivating and brightening an electrodeposited zinc coating, the step comprising subjecting the electrodeposited coating to the action of a bright dip comprising a solution of hydrogen peroxide and sulfuric acid,

RICHARD O. HULL- 

